Plasmaspheric Ion Refilling Rates

Recently there has been intense interest in estimation of the equatorial mass density based on measurements of field line resonance frequencies with ground magnetometers. Using cross-phase analysis of magnetometer array data, we determined the equatorial mass density during a moderate geomagnetic storm that followed several days of magnetically quiet conditions. Early in the storm recovery phase (Kp = 6+), the field line eigenfrequency over L = 2.3 - 3.8 was unusually high, corresponding to very low mass densities and flux tube depletion. Over the next few days the eigenfrequency at these L values progressively decreased, indicating refilling of the flux tube to pre-storm levels, superimposed upon diurnal variations. By comparing density measurements we determined the daytime ion refilling rates and fluxes at the 1000 km level for the L = 2.3, 2.6, 3.3, and 3.8 flux tubes. The upward ion fluxes decreased with increasing L-value, being around 4 x 108 amu/cc/sec at L=2.3 and 1.5 x 108 amu/cc/sec at L=3.8, respectively. We compare these observations with daytime upward electron fluxes measured by VLF whistler techniques, and investigate the L-value dependence in refilling rate. The latter may be attributed to solar zenith angle control of ion production rates.